1. Field of the Invention
The present invention relates to an electroluminescence element (hereinafter referred to as "EL element"), especially a thin-film EL element, used for spontaneously light-emitting segment displays or matrix displays of measuring instruments or displays of various information terminal equipment. The present invention relates also to an element, especially an EL element capable of emitting high luminance light.
2. Description of the Related Art
An EL element utilizes the phenomenon that a fluorescent substance emits light when a high electric field is impressed thereon, and has hitherto become the object of public attention as a product constituting a spontaneously light-emitting flat panel display. As an example of such an EL element, FIG. 7 is a typical view illustrating a typical sectional structure of a conventional EL element. The EL element is formed by laminating a first electrode 2 composed of an optically transparent ITO (Indium Tin Oxide) film, a first insulating layer 3 composed of tantalum pentaoxide (Ta.sub.2 O.sub.5) and the like, a luminescent layer 4, a second insulating layer, and a second electrode 6 composed of a ITO film, in the named order. An ITO film is a transparent conductive film prepared by doping tin (Sn) on indium oxide (In.sub.2 O.sub.3) and has hitherto been widely used for a transparent electrode, because of its low resistivity. In addition, as the luminescent layer 4, there is used, e.g., a product having zinc sulfide as a host material, to which manganese (Mn) or terbium (Tb) has been added as a luminescent center. The luminescent color is determined by the kind of an additive contained in zinc sulfide, and for example, when manganese (Mn) is added as a luminescent center, there is obtained yellowish orange luminescence, and when terbium (Tb) is added as a luminescent center, there is obtained green luminescence.
In EL elements composed of the aforesaid structure, there have been reviewed zinc sulfide (ZnS) with samarium (Sm) as a luminescent center of the luminescent layer capable of obtaining red luminescence therefrom, and zinc sulfide (ZnS) with thulium (Tm) added as a luminescent center of the luminescent layer capable of obtaining blue luminescence therefrom, and so forth. In general, a zinc sulfide (ZnS) luminescent layer with a rare earth metal added is formed by the sputtering method or evaporation method.
However, in a red EL element with samarium (Sm) added or blue EL element with thulium (Tm) added, its luminescence brightness exhibits a very low value of 1000 cd/m.sup.2 (driven at a frequency of 5 KHz ) for red luminescence and of 10 cd/m.sup.2 (driven at a frequency of 5 KHz) for blue luminescence, and under the present situation, such an EL element is poor in practicality for a display such as EL panel. As the reason why sufficient luminance cannot be obtained, there may be considered that, in a luminescent layer formed by the sputtering method or evaporation method, the crystallinity of zinc sulfide (ZnS) with a rare earth element added is poor. That is, in an EL element with samarium (Sm) or thulium (Tm) added, the energy interval between the luminescence excitation level and the level just below is small, as compared with a green EL element with terbium (Tb) added in which a comparatively high brightness has been obtained by use of the sputtering method or evaporation method, so that a non-luminescent multiphonon-emission process tends to compete with and dominate the luminescence transition. Therefore, in an EL element with samarium (Sm) and thulium (Tm) added, it is indispensable to improve the crystallinity of zinc sulfide (ZnS) and reduce the aforesaid non-luminescent transition process. Thus, as a means of improving the crystallinity of zinc sulfide (ZnS) with a rare earth element added, annealing after film formation of a luminescent layer and the like has been performed, but under the present situation, such a means has not exhibited a sufficient effect in zinc sulfide with samarium (Sm) and thulium (Tm) added. As another cause for the lowering of the crystallinity of zinc sulfide (ZnS), there may be mentioned difficulty in replacement of the rare earth element at zinc (Zn) site. The reason therefor is that zinc is different from a rare earth element in ionic radius and valency. For example, the ionic radius of zinc (Zn) is 0.074 nm, whereas that of samarium (Sm) and that of thulium (Tm) are, respectively, 0.096 nm and 0.087 nm, that of the former being different from those of the latter two by approximately twenty to thirty percent. In addition, the valency of zinc (Zn) is divalent, whereas that of a rare earth element is trivalent. Usually, in the luminescent layer of an EL element, a rare earth element is added as a luminescent center in a proportional amount ranging from 0.1 to 1.0 atomic % based on zinc sulfide (ZnS). Consequently, if the zinc is not replaced by the rare earth element to a sufficient degree, a negative influence is exerted upon the crystal lattice, resulting in noticeable lowering of the crystallinity of the luminescent layer. As a result, the acceleration of the carrier in the luminescent layer due to the electric field is impeded, and the probability of the non-luminescent transition process is also increased, resulting in lowering of luminance.
On the other hand, in order to improve the luminescent efficiency and brightness, there has hitherto been proposed a method of controlling the composition ratios of a rare earth element and a halogen element contained in a luminescent layer, as described in, e.g., Japanese Unexamined Patent Publication No. 63-230869. In this literature, there is disclosed a process in which, when forming a luminescent layer composed of a compound of an element belonging to Group II of the Periodic Table and an element belonging to Group VI of the Periodic Table by the sputtering method or evaporation method, a gas containing a halogen element or a halide is used as a reaction gas. However, the process disclosed in this literature of prior art is complicated as a production process, because there are used two kinds of evaporation sources: an evaporation source composed of a compound of an element of Group II and an element of Group VI of the Periodic Table and an evaporation source composed of a sulfide of a rare earth element. In addition, this conventional process is a process in which only the composition ratios of the rare earth element and halogen element contained in a luminescent layer are controlled, and according to this process, there cannot be expected improvement of the crystallinity of a luminescent layer by efficient replacement of a luminescent center element.
In addition, there has been known a report to the effect that, in general, when the crystallinity of a luminescent layer is improved, the luminescence brightness can be also improved. Specifically, with respect to the crystallinity and luminescence characteristics of a luminescent layer of an EL element in the evaporation method, there is known "Multicolorisation and Life-Elongation of Thin-Film EL Element" in Report on the Developments of Researches Vol. 36, No. 6 (1987) P811-818 (or Japanese Journal of Applied Physics, Vol. 26, No. 9, September, 1987, pp1472-1476), and it is therein described to the effect that, in a product containing zinc sulfide as a host material, to which samarium fluoride is added as a luminescent center, (ZnS--SmF.sub.3), with the increase of the concentration of the luminescent center (SmF.sub.3), the diffraction line intensity of the cub-ZnS (111) plane (cub means a cubic structure) per unit film thickness "d", i.e. I .sub.(111) /d is reduced. That is, the more the concentration of a luminescent center becomes increased, the lower becomes the crystallinity of the ZnS--SmF.sub.3 luminescent center. In addition, in, e.g., a report entitled "Crystal Structure Control of ZnS: Tb Film by Bypass Flow Low Pressure CVD Method" in Research Materials of the 6-th EL Sectional Meeting of the 125-th Photoelectric Intertransformation Committee of the Japan Society for the Promotion of Science (Nov. 19, 1991) p7-p14, it is described that the crystal structure in the CVD method can be controlled not only with a carrier HCl gas but with a bypass-flow HCl gas.
However, with respect to a process for the production of an EL element by the sputtering method excellent in productivity among the conventional processes, there has never been reported a specific production process capable of obtaining high luminance, and a practical high luminance EL element by the sputtering method has not been realized. Under such technical circumstances, the present inventors formed a luminescent layer with a fluoride of a luminescent center element added as a luminescent center into a film on zinc sulfide (ZnS) as a host material by the sputtering method, so as to produce an EL element, and measured the X-ray diffraction spectrum of the thus produced EL element by an ordinary X-ray diffraction method, and found that, as illustrated in FIG. 14, a main peak was exhibited at an X-ray diffraction angle (2.theta.) of 28.5.degree.. From this X-ray diffraction result, it was considered that, since the luminescent layer of this element is composed only of a cubic structure (that is, any other crystal structure such as hexagonal structure is not mixed), the crystallinity is very good.
However, when the luminance of this EL element was actually measured, it was found that the actually measured luminance is lower than the luminance expected from the crystallinity. It is presumed from this fact that, in reality, the crystallinity of the luminescent layer will be poorer than the actually measured crystallinity.